Seyfert Galaxies Encyclopedia of Astronomy & Astrophysics
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eaa.iop.org DOI: 10.1888/0333750888/1584 Seyfert Galaxies Mark Whittle From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Thu Mar 02 23:38:01 GMT 2006 [131.215.103.76] Terms and Conditions Seyfert Galaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Seyfert Galaxies are thought to be instrumental in funneling gas down to the nuclear regions where it can then fuel the central black In many respects Seyfert galaxies are similar to any hole. other galaxy, except their nuclear regions show unusual properties: they can be much brighter and their spectra Discovery, classification and detection show strong emission lines of high excitation. Such nuclei The early identification of nearby Seyfert galaxies rested are called ‘active’, and so Seyfert galaxies belong to the on two observational properties—their unusual nuclear wider category of ‘ACTIVE GALAXIES’ or, more specifically, spectra and their unusual nuclear brightness. While they have ‘ACTIVE GALACTIC NUCLEI’ (AGN). Seyferts were, studying galaxy spectra for his PhD work in 1908, Fath in fact, the first type of active galaxy to be discovered. noted strong emission lines in the spectrum of the nucleus In the zoo of AGNs the power of the nuclear activity of NGC 1068. These observations were confirmed in 1917 spans a wide range. Within this zoo, Seyferts show by Slipher and by 1926 Hubble had added two more intermediate levels of activity, being more powerful than galaxies with similar spectra: NGC 4051 and NGC 4151. In LINER galaxies but less powerful than QUASARS. Seyferts 1932 Humason noted that NGC 1275 had a bright starlike also join the majority of active galaxies in being ‘radio- center, but it wasn’t until 1943 that CARL SEYFERT recognized quiet’, with radio luminosities 103–104 times weaker than a distinct class of galaxies with unusually bright and the ‘radio loud’ category, which includes RADIO GALAXIES concentrated nuclei, and studied in detail six of the 12 and radio quasars. then known cases (NGC 1068, 1275, 3516, 4051, 4151, Over the past 20 years much effort has gone into 7469), leading to the group’s name: Seyfert galaxies (see finding Seyfert galaxies amongst the more normal galaxy figure 1). Seyfert’s spectra showed emission lines which population. Efficient ways to detect Seyferts include were not only unusually strong, but also unusually wide. looking specifically for galaxies with bright UV emission, Interpreting the width of the spectral lines as Doppler bright x-ray emission, or unusual far-infrared colors, since shifts caused by motion of the ionized (line emitting) gas these are common characteristics of Seyferts. The fraction implied gas velocities in some objects of several thousand − of galaxies with active nuclei depends on the level of km s 1, far higher than velocities found in normal galaxies. activity: moderate luminosity Seyferts make up ∼ 1–2% of A further subtlety Seyfert noticed was that in some objects nearby bright galaxies, lower luminosity Seyferts make up the hydrogen lines were broader than the other lines. ∼ 10%, while LINERs make up ∼ 40%. Currently, about Little work was done until the 1960s, when it became 800 Seyferts are known, though the number continues to clear that Seyfert galaxies shared a number of properties rise. with the then recently discovered quasars. At this time, From a wide range of studies, a picture has emerged of quasars posed a serious theoretical puzzle. While their the structure of the central regions of Seyferts. Briefly, the high REDSHIFTS implied great distance and therefore high gravitational field of a supermassive (106–109 M) black luminosity, their brightness had been found to vary from hole provides the ultimate energy source, as gas falls month to month, indicating that the region producing inwards via a luminous accretion disk (see SUPERMASSIVE all this energy was only a few light months across—tiny BLACK HOLES IN AGN). A small region of fast moving clouds compared to the rest of the galaxy. The puzzle of how so surrounds this central engine, spanning a few light days much energy could emerge from so small a volume was or weeks, while a larger region of slower moving clouds sufficiently acute that some astronomers even questioned extends out a few hundred to a few thousand light years. whether the quasar redshifts were true indicators of In some cases, oppositely directed collimated jets of low distance, leading to the so-called ‘redshift controversy’. density gas plow into the surrounding galaxy, accelerating Although this controversy has since been resolved in the interstellar medium and creating jet-like or linear favor of large distances it might never have been as radio structures. In at least some Seyferts, dense gas and problematic if Seyfert galaxies had been better studied dust shroud the innermost regions, hiding them from our in the early 1960s, since they are clearly low redshift direct view, though the nuclear radiation escapes in other counterparts to the quasars. Not only are their spectra directions to light up gas and dust residing much further basically similar, but in 1968 NGC 4151 was found to vary in out in the galaxy. brightness, confirming a highly compact nuclear energy The conditions that give rise to Seyfert activity source. As more Seyferts and quasars were discovered, are still under debate. While Seyferts are found in a their separation in luminosity narrowed and finally even wide range of galaxy types, they are more commonly overlapped. Continuity had been established and, for found in reasonably luminous early-type spirals (e.g. most astronomers, the reality of the quasar distances and types S0/a, Sa, Sb in the Hubble classification scheme). luminosities was no longer in doubt. It seems, therefore, that prerequisites favoring Seyfert In 1974, Khachikian and Weedman identified two activity include a massive galaxy bulge and the presence types of Seyfert galaxy on the basis of the widths of the of an interstellar medium. There is also evidence that nuclear emission lines. Figure 2 shows example spectra Seyferts prefer galaxies with an asymmetric gravitational of each Seyfert type. While spectra of type 2 Seyferts field arising from a bar or other distortion, possibly have a single set of relatively narrow emission lines, in induced by a nearby companion galaxy. Such distortions the spectra of type 1 Seyferts the hydrogen and helium Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Seyfert Galaxies E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Figure 1. Three photographs of NGC 4151 taken with increasing exposures (a, b, c) showing its unusually bright nucleus—an important characteristic used in the early identification of Seyfert galaxies. (Taken from Morgan W W 1968 Astrophys. J. 153 27) lines have an additional much broader component. In Seyferts was that of Markarian and collaborators at the the simplest cases, the broad component is either absent Byurakan Observatory inArmenia, during the period 1962 (Seyfert 2) or strong and dominant (Seyfert 1). With to 1981. This survey used a technique known as Objective better data it became clear that there is a wide range in Prism Spectroscopy to find galaxies with unusually blue the relative strength of the broad and narrow emission continuum emission. The telescope was a 1.3 meter lines, and this led Osterbrock in 1981 to refine the Seyfert Schmidt telescope with a thin prism bolted over the classification by introducing intermediate types. A good entrance aperture. This combination gives relatively wide place to establish the classification is with the hydrogen Hβ field photographs in which each star and galaxy appears as line at rest wavelength 4861 Å. As the broad component a small spectrum. In this way, the spectra of many galaxies of Hβ becomes weaker relative to the narrow component, can be studied quickly, and those which appear unusual the Seyfert type changes from 1 to 1.2 to 1.5 to 1.8. For noted and listed. The survey covered ∼ 10 000 square Seyfert 1.8 galaxies, weak broad wings are just visible at degrees and yielded 1500 galaxies with blue continua, of the base of Hβ while for Seyfert 1.9 galaxies they are only which ∼ 10% were Seyferts (the remainder were mainly visible on the Hα emission line at 6563 Å. In practice, STARBURST GALAXIES, blue because of the presence of young these Seyfert sub-types have not been formally defined but massive O and B type stars). Because of the emphasis instead give an overall indication of the degree to which on blue continuum, the Markarian Seyferts contain a the broad component is present. relatively high proportion of Seyfert 1s (about 50%) and Although there are a number of Seyfert properties that rather luminous Seyfert 2s. depend on Seyfert type, perhaps the most noticeable is Since x-ray emission is common in active galaxies, the ‘white light’ continuum coming from the nucleus. In x-ray surveys provide an effective means to find Seyferts. normal galaxies the nuclear continuum is mostly starlight, Starting in the late 1970s surveys have included those of the but in active galaxies there can be additional emission, Ariel V, HEAO-1, Einstein and ROSAT satellites. Some of often blue in color and with few or no absorption features. these were all-sky surveys, some included deeper pointed This component is much stronger in Seyfert 1s, weaker observations, some were sensitive to hard x-rays (2–10 in Seyfert 2s, and may even be undetectable in the lowest keV) and others to soft x-rays (∼ 0.2–4 keV).